The physical phenomena pertaining to electricity and magnetism is described by Maxwell's equations, shown below: EQU .gradient..multidot.D=.rho. EQU .gradient..times.E=-.differential.B/.differential.t EQU .gradient..multidot.B=0 EQU .gradient..times.H=J+.differential.D/.differential.t EQU D=.epsilon.E EQU B=.mu.H
wherein D is the electric displacement, E is the electric field, B is the magnetic flux density, H is the magnetic field, .rho. is the charge density, J is the current density, .epsilon. is the permittivity and .mu. is the magnetic permeability.
Various sources of magnetic fields generally operating at low frequencies are encountered commonly in everyday life. The sources include but are not limited to electric power transmission and distribution lines, transformers, building wiring and such appliances as hair dryers, electrical blankets, video display terminals, electrical ovens, radios and telephones. The magnetic fields generated by such common sources pass through the human body, without significant attenuation since the magnetic permeability of the human body is similar to that of air, i.e. 4.pi..times.10.sup.-7 H/m. The response of living tissue to the magnetic fields passing through is not well understood. While it may take a number of years before scientific studies demonstrate the absence or presence of health effects from low frequency magnetic fields, public concern nevertheless presently exists. In response to this concern, this invention uses shielding to reduce human exposure to these magnetic fields.
In conventional techniques employed to reduce exposure to low frequency, i.e. less than 100 KHz, magnetic fields, the area to be shielded from the magnetic field existing outside is enclosed completely (J. D. Jackson, "Classical Electrodynamics", 2nd edition, John Wiley, NY, 1975, p. 199) or partially by a magnetic material of high relative permeability. Magnetic materials exhibit magnetic permeability values which are significantly greater than the permeability of free space. Such high permeability materials offer a path of low reluctance to the magnetic field lines and shunt the magnetic field lines. Thus, with a magnetic enclosure the magnetic filed magnitude within the enclosure is reduced when the source exists at the outside. However, such mitigation of the magnetic fields by completely enclosing large structures where the magnetic field needs to be reduced, is extremely costly. If it is desired to shield a currentbearing device of small dimensions, the device can again be completely or partially enclosed by a magnetic material of high relative permeability (W. R. Smyth, "Static and Dynamic Electricity", 2nd edition, McGraw-Hill, NY, 1950, p. 288). However, this method is not practical for cases where the source is large in dimensions and extends over long distances, A good example of such a magnetic field source is a high voltage electric power transmission line.
Electromagnetic interference shielding covers for shielding objects, such as radios, from receiving travelling electromagnetic waves are described, for example, in U.S. Pat. No. 4,474,676 and for shielding computer terminals and the like from emitting electromagnetic fields, are described in U.S. Pat. No. 4,785,136.
Accordingly, it is an object of the invention to mitigate the magnetic field of a magnetic field generating source at a minimum of cost.
It is another object of the invention to mitigate the magnetic field of a structure such as an electric power transmission line and a distribution line.
It is another object of the invention to provide a magnetic field mitigation body with an external field which can reduce the field of the source in an extended area that is large as compared to the size of the shield.
Briefly, the invention provides a method and apparatus for mitigating a magnetic field from a magnetic field source.
In accordance with the method, the pattern of a magnetic field generated about a magnetic field generating source is first determined and, thereafter, a body of magnetic material is positioned adjacent to the source in a predetermined area of the magnetic field pattern in a manner to reduce the magnitude of the pattern at an area remote from the source and the body, that is, at the area which is to be protected.
The method is particularly directed to altering the magnetic field generated by a source operating at a low frequency, typically less than 100 KHz, and by using a magnetic body of proper shape, size and physical properties. Thus, the method is also applicable to frequencies other than those used for electric power (50-60 Hz).
The magnetic body may consist of various materials, including pultruded strips of soft magnetic materials, which posses high permeability values, strips or assemblies of such magnetic materials, extruded profiles or molded bodies of magnetic composites consisting of soft magnetic fillers and conductive or non-conductive binders/matrices. The magnetic body used to mitigate the magnetic field generated by a source may be solid, hollow or of mesh design. The shape and size of the magnetic body depend on the magnetic field generated by the source, the physical properties of the magnetic body and the targeted magnetic field at specified locations. The method for the optimization of the size and shape of the magnetic body involves the detailed solution of Maxwell's equations, analytically or by employing numerical techniques. Experimental techniques may also be used for insight and validation of results.
It should be noted that the possibility of the mitigation of the magnetic field at locations far away from a source by placing a magnetic body into the vicinity of the source is not obvious to those skilled in the art. The classical problem of placing a cylinder with an infinite relative permeability into the vicinity of a single current carrying wire (E. Weber, "Electromagnetic Fields", Vol. 1, John Wiley, NY, 1950, p. 240) in the most favorable position of the cylinder touching the wire is an example. Little change in the magnetic field at far-away locations are observed associated with the presence of the cylinder with infinite permeability. If the radius of the cylinder is 5% of the distance of separation between the wire and the location where mitigation is desired, the magnetic field would be reduced to only 95% of the original field due to the presence of this relatively large cylinder.
After the body of magnetic material is placed in the vicinity of the magnetic field source, the body becomes magnetized under the influence of the source field. Then, the field of the magnetic material alters the field of the original source, reducing the field in some regions and enhancing the field at other locations. If the shape, physical properties and the size of the magnetic body are selected carefully, the size of the magnetic body can be kept comparable to the characteristic dimensions of the source and still mitigate the field at distances which are long compared to the characteristic dimension of the magnetic body.
In the case of electronic devices or other sources of magnetic fields, the use of a body of magnetic material close to the source may eliminate the need to completely or partially enclose the source of the magnetic field.
Mitigation of time varying magnetic fields requires a magnetic material which can change its direction of magnetization in response to the applied field. The magnetization of the material must follow the external field with a reasonable degree of efficiency. Such materials are called soft magnetic materials. The magnetic properties of some soft ferromagnetic alloys, which are commercially available, are given below. The magnetic permeability of the material is a measure of how well the material can become magnetized with the application of the external field. The coercive force is the magnetic field at which the magnetic flux density becomes zero. The saturation flux density of the material is the field up to which the material can be used without degradation of its magnetic permeability. High magnetic permeability high saturation flux densities and small coercive forces are desirable for this application.
______________________________________ MAXIMUM SATURATION MAGNETIC COERCIVE FLUX TRADE PERME- FORCE DENSITY NAME ABILITY (OERSTEDS) (KILOGAUSS) ______________________________________ Permalloy* 200,000 0.015 8.0 Supermalloy* 300,000 0.004 7.8 Metglas 600,000 0.015 7.7 2705M** Metglas 1,000,000 0.002 5.7 2714A** ______________________________________ *G.Y. Chin and J.H. Wernick, "Soft Magnetic Metallic Materials" in Editor E.P. Wohlfart, "Ferromagnetic Materials", Vol. 2, Ch. 2, North Holland, NY, 1986, p. 143. **Metglas Technical Bulletin, Allied Signal Metglas Products, Parsippany, NJ.
The above described method may also be used to increase the magnitude or the components of the magnetic field at targeted locations, if desired.
For field mitigation to be effective, the shape and size of the magnetic body should be properly selected. The size and shape of the magnetic body needs to be determined for each case, where mitigation is desired. The tools include the numerical or analytical solutions of Maxwell's equations or experimental studies.
In one embodiment, the apparatus is employed for an electric power transmission system including electrically conductive lines (i.e. conductors) for conducting electrical current with a corresponding magnetic field being produced transversely of each line. Generally, such a system employs two towers for supporting the conductors in an elevated position along and above a predetermined right-of-way. In accordance with the invention, a body of magnetic material is deployed adjacent to at least one of the lines which acts as a magnetic field generating source. This body is so disposed between the conductor and the right-of-way so as to intercept and distort the magnetic field relative to the right-of-way to an extent to reduce the magnitude of the magnetic field at and beyond the right-of-way.
In this system, the electrically conductive lines conductors may be disposed in a rectangular array or any other suitable array commonly used for electric power transmission lines. In accordance with the invention, the magnetic body is disposed adjacent to one of the lowermost conductors and between the conductor and the right-of-way. However, the body may be located adjacent to one of the other conductors and, also, multiple bodies may be used.
Generally speaking, the body is disposed close to the source of the magnetic field rather than to the object to be protected against the magnetic field.
The magnetic field mitigation body serves to mitigate magnetic fields that exhibit a dipole or multipole nature by using the external field of a soft magnetic material placed in the immediate vicinity of the source. In this respect, the body is not required to enclose the source.
The field of the magnetic body is a dipole field. In this respect, the body does not operate effectively against a source that has a monopole field such as a single current-carrying wire. However, as the order of the source field increases, for example from a monopole to a dipole or from a dipole to a quadrupole, the effectiveness of the body increases.
The body may also be employed in the mitigation of magnetic fields generated by sources other than electric power transmission lines and the like.
The shape and size of the body and the location of the body with respect to the source of the magnetic field are chosen such that the field magnitude will be reduced in certain directions.
In accordance with the invention, the magnetic field of the source magnetizes the soft magnetic body and the external field of the magnetized body opposes that of the source in some directions. The geometry and location of the body with respect to the source can be optimized to yield maximum reduction in the desired regions.
One characteristic of the body is that the external field of the body can reduce the field of the source in an extended area that is large as compared to the size of the body.
The shape of the body is chosen so as to conform to an original magnetic field plot in order to achieve optimum magnetization and maximum dipole moment for a given size. Further, the body may be solid, hollow, in the form of a mesh, laminated in layers and the like.